1. Field of the Invention
This invention relates to electrochemical machining (ECM), and particularly to the ECM of fluid dynamic bearing and race surfaces.
2. Description of the Related Art
In the prior art, microfeatures on surfaces, such as the fluid dynamic grooves on bearing and race surfaces, were formed by machining or casting.
Microfeatures on surfaces in many applications involve the machining of very hard materials, such as ceramics or hard metals. These materials are difficult to work with, and often the result of machining the features was undesirable displacement of the material, resulting in "lips" or "spikes", also called "kerf" by many machinists, small eruptions of materials along the edges of the grooves or other features. Removing the undesired material often meant polishing or deburring, while great care is required to avoid damage to the surface with microfeatures.
For example, shallow trenches with a contoured cross-section are often made by pressing on the material with a ball of hard material. This type of manufacturing is known to preferentially pile up material along the edge of the trench. Since the edge of the trench may meet the original surface at a shallow angle, the pile up of material can be very difficult to remove.
Another method of machining grooves involves electrochemical machining. In electrochemical machining, an electrode is applied to a work which may comprise a machined surface piece where it is desired to form a feature. Both the electrode and the work piece are in an aqueous electrolyte containing a salt usually NaNO.sub.3 or NaCl. A current from the electrode to the work piece causes the ions of the workpiece to dissolve in solution.
In the following discussion, etching or removal of material is assumed, but it will be obvious that similar results would be achieved in electrodepositing material, such as in electroplating materials onto a medium.
Etching occurs where the current reaches the work piece. This effect is pronounced enough to allow microfeatures to be etched in the work piece with only the effect of a cleaning operation elsewhere, which is often independently desirable.
Electrochemical etching avoids the problem of material pile up, since there is no physical pushing action to cause a rearrangement of the material, but creates problems of its own. One problem introduced by electrochemical machining is that the electrode requires a finite time for material removal. The electrode is often complex and difficult to make. In one prior art approach to making grooves for a bearing, this insulation is placed on the surface to be grooved. This makes for slow, tedious work, and greatly increases costs while limiting productive throughput.
The present invention addresses the above problems, and introduces benefits not discussed in the interests of brevity and clarity in the discussion above.